1. Field of the Invention
The present invention relates to a monitor, and more particularly to a horizontal deflection circuit for a monitor capable of stabilizing an amplitude of distortion correcting signal of parabolic waveform supplied to a horizontal deflection coil.
2. Description of the Prior Art
A conventional monitor is an apparatus which displays a video signal in accordance with an externally-supplied synchronization(hereafter, simply referred to as "sync") signal. The above conventional monitor separates a deflecting circuit for generating a horizontal deflection signal of sawtooth waveform and for supplying the horizontal deflection signal to a horizontal deflection coil and a distortion correcting circuit for generating a distortion correcting signal of parabolic waveform and for supplying the distortion correcting signal to the horizontal deflection coil.
However, in order to enhance a quality of a picture, the deflecting circuit and the distortion correcting circuit includes in the conventional horizontal deflection circuit.
Generally, an electron emitted from a cathode of a Cathode Ray Tube(CRT) is accelerated in accordance with a voltage applied to a grid terminal and then is deflected by a magnetic field generated at a deflection coil. The deflected electron collides with a phosphor screen and the point of collision of the phosphor screen is moved.
The deflecting distance is proportional to the intensity of the magnetic field and the intensity of the magnetic field is proportional to the quantity of current of sawtooth waveform which flows in the deflection coil. Therefore, the multi-mode monitor is equipped with a horizontal deflection coil for deflecting the electron in the horizontal direction from left to right constantly and returning it from right to left rapidly and a vertical deflection coil for deflecting it in the vertical direction.
FIG. 1 is a view for showing a horizontal deflection signal of sawtooth waveform supplied to a horizontal deflection coil in a conventional monitor. Here, the electron is deflected to the leftmost end point of the phosphor screen at a point a. Since the current does not flow to the horizontal deflection coil, the electron advances straight to a point b. Further, since the quantity of current gradually increases, the electron is deflected to the rightmost end point of the phosphor screen at a point c. Thereafter, the quantity of current is suddenly reduced and the electron is again deflected to the leftmost end point at a point a1.
FIG. 2 is a view for showing a construction of a conventional horizontal deflection circuit for a monitor. Here, reference numeral 1 denotes a deflection section for generating a horizontal deflection signal of sawtooth waveform in accordance with an externally-supplied horizontal sync signal and for supplying the horizontal deflection signal to a horizontal deflection coil HDY.
Reference numeral 2 denotes a distortion correcting section for superimposing a horizontal picture size signal and a distortion correcting signal corresponding to a mode of the monitor such as a video graphic adapter(VGA), super VGA (SVGA), etc., which determines according to the horizontal sync signal and the vertical sync signal , and for supplying a superimposed signal to the deflection section 1.
The deflection section 1 comprises an oscillator 11 for oscillating and pulse-width modulating the externally-supplied horizontal sync signal in accordance with the frequency thereof and for outputting an oscillating signal, a horizontal driving unit 13 for amplifying the oscillating signal in accordance with a voltage of an externally-supplied voltage source Vcc and for generating a driving signal, and a horizontal output unit 15 for generating a horizontal deflecting signal of sawtooth waveform in accordance with the horizontal driving signal and for supplying the horizontal deflecting signal to the horizontal deflection coil HDY.
In the meantime, the distortion correcting section 2 includes a microprocessor 21 for determining the mode of the monitor according to a frequencies of the horizontal sync signal Hs and a vertical sync signal Vs and for generating a control signal to generate a horizontal picture size signal and a distortion correcting signal of parabolic waveform, a distortion correcting signal generating unit 22 for superimposing the distortion correcting signal and the horizontal picture size signal, a pulse-width modulating(hereafter simply referred to as PWM) signal generating unit 24 for pulse-width modulating the superimposed signal, and for outputting the PWM signal, a buffer 25 for buffering the PWM signal, and a distortion correcting signal output unit 27 for amplifying the PWM signal in accordance with an output signal of the buffer 25 and an externally-supplied voltage source B+ and for outputting an amplifying signal to the horizontal deflection coil HDY.
In the above described construction of the conventional horizontal deflection circuit for a monitor, the horizontal sync signal Hs is supplied to the oscillator 11 of the deflection section 1, and the oscillator 11 converts a horizontal sync signal Hs to the oscillating signal. The oscillating signal is supplied to the horizontal driving unit 13 which amplifies the oscillating signal and generates a driving signal. The driving signal is supplied to the horizontal output unit 15. The horizontal output unit 15 generates the horizontal deflection signal of sawtooth waveform which flows in the horizontal deflection coil HDY in accordance with the amplifying signal supplied from the distortion correcting signal output unit 27 and the externally-supplied voltage source B+, as shown in FIG. 1.
The horizontal sync signal Hs and the vertical sync signal Vs are supplied to the microprocessor 21 of the distortion correcting section 2, and the microprocessor 21 determines the mode of monitor in accordance with the horizontal sync signal Hs and the vertical sync signal Vs and generates the control signal to be inputted to a distortion correcting signal generating unit 22 for generating the horizontal picture size signal and the distortion correcting signal of parabolic waveform.
The distortion correcting signal generating unit 22 superimposes the horizontal picture size signal and the distortion correcting signal, and the superimposed signal is supplied to the PWM signal generating unit 24. The PWM signal generating unit 24 modulates the superimposed signal and outputs the PWM signal, as shown in FIG. 3.
The PWM signal is supplied to the buffer 25 which buffers the PWM signal. The output signal of the buffer 25 is supplied to the distortion correcting signal output unit 27.
The distortion correcting signal output unit 27 amplifies the output signal of the buffer 25 in accordance with the externally-supplied voltage source B+ and supplies the amplifying signal to the horizontal deflection coil HDY.
However, in such conventional horizontal deflection circuit, when the mode is changed, the distortion correcting signal is unstable, and the unstable distortion correcting signal is supplied to the horizontal deflection coil. Therefore, the horizontal deflection circuit is unstable and a switching device of the distortion correcting output unit is damaged by a switching cycle of the PWM signal.